Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS4353037 A
Publication typeGrant
Application numberUS 06/176,628
Publication dateOct 5, 1982
Filing dateAug 11, 1980
Priority dateAug 11, 1980
Also published asCA1156320A, CA1156320A1
Publication number06176628, 176628, US 4353037 A, US 4353037A, US-A-4353037, US4353037 A, US4353037A
InventorsGene D. Miller
Original AssigneeMotorola, Inc.
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Amplifier protection circuit
US 4353037 A
Abstract
A protection circuit for a transmitter amplifier which provides power leveling and controls transmitter output power as a function of the ratio of reflected power to forward power. The circuit senses forward power level and develops a first voltage which is compared with the reference voltage to provide a control voltage which controls the power developed by the transmitter amplifier. Reflected power is sensed and a second voltage is developed which is compared to a portion of the first voltage and causes a reduction in the reference voltage when the reflected power to forward power ratio exceeds a predetermined level.
Images(2)
Previous page
Next page
Claims(7)
What is claimed is:
1. A protection circuit, for controlling the power of a transmitter amplifier which produces an output varying in response to a control signal applied thereto, comprising:
sensing means, coupled to the transmitter amplifier, for generating a forward power signal representative of the forward power and reflected power signal representative of the reflected power;
first comparator means, coupled to the sensing means for generating a reference voltage at an output, and for reducing the reference voltage to a predetermined value in response to the ratio of the reflected power to the forward power exceeding a predetermined level; and
second comparator means, having a first input coupled to the output of the first comparator means and a second input coupled to the sensing means, for generating a control signal which varies in accordance with the difference between the reference voltage and the forward power signal, said second comparator means being coupled to the transmitter amplifier for applying said control signal thereto to vary the power developed by the transmitter amplifier.
2. The protection circuit of claim 1 wherein the first comparator means comprises:
first differential amplifier means, having a non-inverting input for receiving the reflected power signal and an inverting input for receiving a predetermined portion of the forward power signal, and having an output signal which is low when the reflected power signal is less than the predetermined portion of the forward power signal and high when the reflected power signal is greater than the predetermined portion of the forward power signal; and,
amplifier means coupled to the output of the differential amplifier means, for generating at an output, a nominal reference voltage when the differential amplifier means output signal is low, and for reducing the reference voltage to the predetermined value when the differential amplifier means output signal is high.
3. The protection circuit of claims 1 or 2 wherein the second comparator means comprises:
second differential amplifier means, having a non-inverting input coupled to the output of the first comparator means, and having an inverting input coupled to the sensing means, for generating a signal at an output which varies in accordance with the difference between the reference voltage and the forward power signal;
amplifier means, for generating a control signal in response to the signal at the output of the second differential amplifier means; and
coupling means, for coupling the control signal to the transmitter amplifier.
4. The protection circuit of claims 1 or 2 wherein the sensing means comprises a directional coupler.
5. The protection circuit of claims 1 or 2 wherein the reference voltage is adjustable.
6. The protection circuit of claim 3 wherein the sensing means comprises a directional coupler.
7. The protection circuit of claim 3 wherein the reference voltage is adjustable.
Description
BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates to amplifier protection circuits and more particularly to a fixed ratio VSWR protection circuit which provides power leveling and VSWR protection.

B. Description of the Prior Art

VSWR mismatch protection circuits are used in RF transmitter circuits to insure that power output circuitry is not damaged when the antenna is uncoupled or when the antenna is suddenly damaged or broken. One type of protection circuit employed provides protection by sensing the current to the final amplifier and reducing the current to a preceding stage with increases in final amplifier current. However, it is possible for a transmitter amplifier to be connected to an improper load, such as an open circuit, and still draw the required amount of current. In such a case, the amplifier will dissipate both the power developed, called "forward power", and the power reflected by the improper load, called the "reflected power". If the amplifier is not capable of dissipating the combined forward and reflective power, it can be seriously damaged before any increase in the amplifier current is sensed.

Another type of protection circuit commonly used senses increases in reflected power, and decreases the power of the transmitter in response. Such circuits do not protect against increases in forward power of the transmitter amplifier due to variations in supply voltage and circuit characteristics, which can cause overdissipation of the transmitter amplifier. Some of these prior art circuits continuously cut back the gain of the transmitter amplifier in a linear fashion such that the overall output gain of the amplifier can be less than optimum even though protection is not required. In addition, since the reflected power may increase with an increase in forward power and no allowance is made in the protection circuit for increased forward power, the reflected power sensing mechanism of such prior art circuits can falsely cut back the gain of the transmitter amplifier when the forward power is increased. Thus, it is desirable to provide a protection circuit which controls the output power of a transmitter based upon the ratio of the reflected power to the forward power when that ratio exceeds a certain dangerous threshold.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a protection circuit for a transmitter amplifier which controls transmitter output power as a function of the ratio of reflected power to forward power.

It is another object of this invention to provide a protection circuit for a transmitter amplifier which maintains a predetermined power level until a predetermined ratio of the reflected power to forward power is exceeded.

It is still another object of this invention to provide a protection circuit for a transmitter amplifier which permits forward power to be adjustable.

In practicing the invention, a protection circuit is provided for a transmitter amplifier which senses variation in forward and reflected power and maintains a safe forward power level. A directional coupler and detector circuit is coupled to the transmitter output to act as a sensing circuit to sense the forward and reflected power level and to generate a forward power signal representative of the forward power and a reflected power signal representative of the reflected power. A portion of the forward power signal and the reflected power signal are coupled to a first comparator circuit. The first comparator circuit generates a reference voltage which is reduced in response to the ratio of reflected power to the forward power exceeding a predetermined level. The reference voltage and the forward power signal are coupled to a second comparator means. The second comparator means generates a control signal which varies in accordance with the difference between the reference voltage and the forward power signal. This second comparator means is coupled to the transmitter amplifier such that the control signal varies the power developed by the transmitter amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, together with further objects, features and advantages thereof, may best be understood by reference to the following description when taken in conjunction with the accompanying drawings.

FIG. 1 is a generalized block diagram of the novel protection circuit according to the invention.

FIG. 2 is a detailed schematic diagram of the preferred embodiment of the novel protection circuit according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a generalized block diagram of a protection circuit 5 constructed in accordance with the present invention. Modulated RF signals are coupled to a transmitter amplifier 9 via an RF input 190 where they are amplified to the desired RF power level and coupled to an antenna 202 via a directional coupler and detector circuit 11. The antenna 202 radiates the RF signal so that it can be picked up by desired receivers.

The RF power developed by the transmitter amplifier 9 and coupled to the antenna 202 is commonly termed "forward power". As the transmitter amplifier 9 and the antenna 202 are not electrically ideal elements, part of the forward power coupled to the antenna 202 will be reflected back to the transmitter amplifier 9. If the antenna 202 should accidentally be broken or short circuited, all of the RF power will be coupled from the antenna 202 back to the transmitter amplifier 9. The RF signal reflected from the antenna 202 back to the transmitter amplifier 9 is commonly termed "reflected power".

Forward power developed by the transmitter amplifier 9 and reflected power coupled back to the transmitter amplifier 9 are sensed by the directional coupler and detector circuit 11, which develops a forward power signal VFWD representative of the forward power, and a reflected power signal VREFL representative of the reflected power. The forward power signal VFWD is coupled to an input terminal 14 of the protection circuit 5 and the reflected power signal VREFL is coupled to another input terminal 12 of the protection circuit 5, as shown.

The forward power signal VFWD is coupled from the input terminal 14 to the input 34 of a comparator stage 30. The comparator stage 30 comprises a differential or operational amplifier with appropriate feedback (not shown). A reference voltage VREF is coupled to the input 32 of the comparator stage 30 from the output 28 of a reference voltage circuit 26. The comparator stage 30 constantly compares the forward power signal VFWD to the reference voltage VREF and develops a signal proportional to the difference between the reference voltage VREF and the forward power signal VFWD at its output 36. The signal at the output 36 of the comparator stage 30 is applied to the input 38 of a control voltage circuit 40 which amplifies the signal to produce a control voltage at its output 42. This control voltage is then coupled from the output 42 of the control voltage circuit 40 to the transmitter amplifier 9 so as to control the power output.

Since the output voltage of the comparator stage 30 is proportional to the difference between the reference voltage VREF and the forward power signal VFWD, the control voltage is also proportional to that difference. As a result, any change in the output power (and therefore VFWD) results in a change in the control line voltage which returns the output power to its original value. As an example, if the output power drops due to a drop in the supply voltage, the VFWD will also drop. This will result in an increase in the difference between VFWD and VREF causing the control voltage to increase. The increased control voltage will cause the transmitter amplifier 9 to increase the power output to its original value. Thus, this part of the circuit provides power leveling by automatically adjusting the control line voltage so that the forward power signal VFWD is approximately equal to the reference voltage VREF. In the preferred embodiment, the reference voltage circuit 26 allows VREF to be adjusted, thereby permitting the output power to be adjustable.

To provide protection against excessive reflected power, the reference voltage VREF is reduced, thus reducing output power, whenever the ratio of reflected power to forward power exceeds a preset threshold. This reduction of the reference voltage is controlled by a comparator stage 20. The reflected power signal VREFL is coupled from the input terminal 12 to the input 13 of the comparator stage 20. The comparator stage 20 comprises a differential or operational amplifier with appropriate feedback (not shown). The forward power signal VFWD is applied to a second input 18 of the comparator stage 20 via the resistor network composed of the resistors 16 and 17. The resistors 16 and 17 form a divider network which couple a predetermined portion of the forward power signal VFWD to the input 18 of the comparator 20. As a result, the comparator stage 20 compares the reflected power signal VREFL to a percentage of the forward power signal VFWD. The percentage of VFWD applied to the comparator stage 20 is determined by the value of the resistors 16 and 17 and the gain of the comparator stage 20. The output 22 of the comparator stage 20 is coupled to the input 24 of the reference voltage circuit 26.

The output 22 of the comparator stage 20 will remain in a low state as long as the ratio of reflected power to foward power is less than the threshold level determined by resistors 16 and 17, and the gain of the comparator stage 20. As long as the output of the comparator stage 20 is in a low state, the reference voltage circuit 26 will generate a predetermined nominal reference voltage at its output 28. However, when the reflected power signal VREFL becomes large enough to exceed threshold, the output 22 of the comparator stage 20 goes high, causing the reference voltage at the output 28 of the reference voltage circuit 26 to reduce to some fixed fraction of the nominal level. Reducing the reference voltage causes the control voltage applied to the transmitter amplifier 9 to be reduced by the action of the comparator stage 30. Thus, the power output of the transmitter amplifier 9 is reduced to a safe level.

Referring now to FIG. 2, there is shown a more detailed schematic diagram of the preferred embodiment of the novel protection circuit according to the invention. Modulated RF signals are applied to an input terminal 190 and coupled to an RF control amplifier 192. The amplified signals are then coupled to an RF power amplifier 198 and the resulting output signal is coupled to an antenna 202 via a directional coupler 200. An RF detector circuit 204 is coupled to the directional coupler 200 as shown. A forward power signal VFWD and the reflected power signal VREFL, are coupled through the directional coupler 200 and the RF detector 204 to the input terminals 112 and 114 of the protection circuit 10. This forward power signal VFWD is then applied to the inverting input 156 of a comparator stage 150 via resistors 119 and 154, as shown. The comparator stage 150 is preferably a differential or operational amplifier. A resistor 117 serves as a load resistor for the forward power output 206 of the RF detector 204. A resistor 121 is coupled from the terminal 114 to the supply voltage to provide necessary bias to insure proper initial conditions. The comparator stage 150, transistors 162 and 172, together with resistors 154, 158, 168 and 180, form a high gain comparator circuit. The resistors 119, 186 and a capacitor 184, connected as shown, form a negative feedback network which establishes the loop gain and frequency response of the comparator circuit while providing immunity to gain variations. A reference voltage VREF is coupled through a transient filter composed of a resistor 146 and a capacitor 148, to the non-inverting input 152 of the comparator stage 150 via the conductor 144, as shown. The high gain comparator circuit composed of comparator stage 150 and transistors 162 and 192, constantly compares the forward power signal VFWD to the reference voltage VREF, and applies to the control line 181 a control signal proportional to the difference between the reference voltage VREF and the forward power signal VFWD. The signal is coupled via the control line 181 to the control input 194 of the RF control amplifier 192. Since the control voltage is proportional to the difference between the reference voltage VREF and the forward power signal VFWD, the change in output power, and therefore in VFWD, results in an opposite change in the control line voltage. This returns the output power to its original value. Thus, power leveling is providing by changing the control line voltage to keep the forward power signal VFWD approximately equal to the reference voltage VREF.

The reflected power protection is provided by reducing output power, by controlling the value of the reference voltage VREF, when the ratio between reflected power and forward power exceeds a predetermined threshold. The reflected power signal VREFL from the output 208 of the RF detector 204 is coupled to the terminal 112 and from the terminal 112 to the non-inverting input of a comparator stage 120. In the preferred embodiment, the comparator stage 120 is a differential or operational amplifier. The comparator stage 120 and the resistor network composed resistors 113, 115, 122, connected as shown form a comparator with hysteresis. The hysteresis is introduced in order to overcome non-linearities inherent in the directional coupler detector circuit. The resistor 113 also serves as a load resistor for the reflected power output 208 of the RF detector 204. The forward power signal VFWD is coupled from the terminal 114 to the inverting input of the comparator stage 120 via the resistor network composed of resistors 116 and 118, as shown. This permits the comparator stage 120 to continually compare the reflected power signal VREFL to a predetermined fraction of the forward power signal VFWD, where the predetermined fraction is determined by the value of the resistors 116 and 118 and by the gain of the comparator. The output of the comparator stage 120 will remain low as long as the reflected power signal VREFL is less than the predetermined fraction of the forward power signal VFWD. The exact relationship is given by the following expression: ##EQU1##

As long as the expression (1) is valid, the output 124 of the comparator stage 120 will be low. This low voltage is coupled to the base 132 of a transistor 130 through a resistor network composed of resistors 126 and 128, thereby maintaining the transistor 130 in the "off" state. Coupled to the collector 136 of the transistor 130 is a resistor network composed of resistors 140, 138 and variable resistor 142 as shown. When the transistor 130 is in the "off" state, the voltage at the node 139 will be a maximum. This voltage is the nominal reference voltage VREF and is determined by the voltage divider ratio of the resistor 140 and the variable resistor 142. This reference voltage VREF, is coupled via the conductor 144 to the input 152 of the comparator stage 150 through the low-pass transient filter composed of resistor 146 and capacitor 148. By varying the position of the wiper 141 of the variable resistor 142, the reference voltage VREF can be adjusted, permitting the power output to be adjustable. Thus, it can be seen from the above description, an adjustable, nominal reference voltage is generated when the reflected power signal is less than the threshold value given by expression (1). However, if the reflected power increases such that expression (1) is not true, then the output 124 of the comparator stage 120 goes "high". The resistors 126 and 128 are chosen so that a "high" causes the transistor 130 to saturate. The reference voltage VREF at the node 139 then becomes a lower value determined by the resistor network composed of resistors 140, 138 and the variable resistor 142. Since this new reference voltage (cutback reference) is less than the initial nominal reference voltage, power output will be cutback. By proper selection of the resistors 138, 140 and 142, the cutback power level can be made to be the same percentage of the nominal power setting (as determined by the position of the wiper 141) over a wide range of nominal power settings. In addition, since the cutback reference voltage is a fixed value, the reduced forward output power will be power leveled even in the cutback mode.

From the foregoing description it can be seen that a protection circuit has been provided which controls output power based upon the ratio of reflected power to forward power. In addition, the circuit provides power leveling even in the reduced power mode and permits the nominal output power to be adjustable.

While a preferred embodiment of the invention has been described and shown, it should be understood that other variations and modifications may be implemented. It is therefore contemplated to cover by the present application any and all modifications and variations that fall within the true spirit and scope of the basic underlying principles disclosed herein.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3366883 *Dec 20, 1965Jan 30, 1968Avco CorpAutomatic broad band vswr power control
US3641451 *Feb 24, 1970Feb 8, 1972Motorola IncAmplifier protection circuit
US3652898 *Dec 27, 1968Mar 28, 1972Combustion EngDual channel monitoring apparatus
US3852669 *Jun 26, 1973Dec 3, 1974Us ArmyCircuit to protect rf output amplifier against mismatch damage
US3870957 *Oct 15, 1973Mar 11, 1975IttVSWR alarm system
US4019150 *Nov 17, 1975Apr 19, 1977Motorola, Inc.PA protection circuit for a single sideband radio
US4114108 *May 19, 1977Sep 12, 1978General Electric CompanyOverdrive protection circuit
US4122400 *Apr 25, 1977Oct 24, 1978Rca CorporationAmplifier protection circuit
SU517988A1 * Title not available
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4521912 *Jun 9, 1983Jun 4, 1985General Electric CompanyLow power indicating circuit for a radio transmitter
US4709404 *Aug 13, 1985Nov 24, 1987Nec CorporationBattery-powered radio communication apparatus capable of prolonging the communicable state thereof
US4819272 *Mar 13, 1987Apr 4, 1989Sony CorporationIntermittently driven transmitter
US4859967 *Nov 9, 1988Aug 22, 1989Harris CorporationRF power amplifier protection
US5020138 *Mar 28, 1990May 28, 1991Sony CorporationRadio communication apparatus having cooling means
US5196808 *Dec 2, 1991Mar 23, 1993Motorola, Inc.RF amplifier protector and method
US5689267 *Nov 22, 1995Nov 18, 1997Allied Signal IncAutomatic VSWR sensing for aircraft-mounted pulse radar systems
US5913154 *Apr 18, 1997Jun 15, 1999Ericsson, Inc.VSWR control technique for terminal products with linear modulation
US6593814Sep 10, 2001Jul 15, 2003Infineon Technologies AgAmplifier circuit with protective device
US6710651 *Oct 22, 2001Mar 23, 2004Kyocera Wireless Corp.Systems and methods for controlling output power in a communication device
US6791406Feb 19, 2003Sep 14, 2004AlcatelPower servo-loop, an RF signal amplifier circuit, and an RF signal transmitter fitted with such a circuit
US6990323Oct 11, 2002Jan 24, 2006Koninklijke Philips Electronics N.V.RF power amplifier circuit
US7336935 *Jun 20, 2005Feb 26, 2008Inventec Appliances Corp.Dynamic power control circuit of wireless communication device
US7567530 *Dec 6, 2005Jul 28, 2009Pantech Co., Ltd.Method of converting communication channel in mobile communication terminal
US8059702Mar 14, 2007Nov 15, 2011Motorola Mobility, Inc.Monitoring multiple modem transmission in a communication device
US8170604Jun 27, 2006May 1, 2012Motorola Mobility, Inc.Method and system for managing communications for a multi-mode communications device
US8195250Apr 30, 2008Jun 5, 2012Motorola Mobility, Inc.Method and apparatus for controlling power among modems in a multi-mode mobile communication device
US8665778Mar 15, 2007Mar 4, 2014Motorola Mobility LlcMonitoring and control of transmit power in a multi-modem wireless communication device
US8665779Sep 15, 2012Mar 4, 2014Motorola Mobility LlcMonitoring and control of transmit power in a multi-modem wireless communication device
US8731492 *Jul 17, 2008May 20, 2014Freescale Semiconductor, Inc.System for monitoring and controlling the power of a radio frequency (RF) signal in a short-range RF transmitter
US8744519Dec 14, 2006Jun 3, 2014Motorola Mobility LlcMultimodal phone data session management enhancement that alleviates dual transmission problems
US9491723May 15, 2014Nov 8, 2016Freescale Semiconductor, Inc.System for monitoring and controlling the power of a radio frequency (RF) signal in a short-range RF transmitter
US20030076168 *Oct 22, 2001Apr 24, 2003Tim ForresterSystems and methods for controlling output power in a communication device
US20030087626 *Oct 11, 2002May 8, 2003Prikhodko Dmitry PavlovichRF power amplifier circuit
US20030114182 *Jan 16, 2002Jun 19, 2003Chan Paul L.Adaptive power amplifier
US20030155971 *Feb 19, 2003Aug 21, 2003AlcatelPower servo-loop, an RF signal amplifier circuit, and an RF signal transmitter fitted with such a circuit
US20060057979 *Jun 20, 2005Mar 16, 2006Inventec Appliances Corp.Dynamic power control circuit of wireless communication device
US20060120330 *Dec 6, 2005Jun 8, 2006Pantech Co., Ltd.Method of converting communication channel in mobile communication terminal
US20070298835 *Jun 27, 2006Dec 27, 2007Motorola, Inc.Method and system for managing communications for a multi-mode communications device
US20080102874 *Oct 28, 2006May 1, 2008Motorola, Inc.Control of transmit power of a second transmitter based on antenna loading parameters measured on a first transmitter
US20080130727 *Mar 14, 2007Jun 5, 2008Motorola, Inc.Monitoring multiple modem transmission in a communication device
US20080130728 *Mar 15, 2007Jun 5, 2008Motorola, Inc.Monitoring and control of transmit power in a multi-modem wireless communication device
US20080146268 *Dec 14, 2006Jun 19, 2008Motorola, Inc.Multimodal phone data session management enhancement that alleviates dual transmission problems
US20090275355 *Apr 30, 2008Nov 5, 2009Motorola, Inc.Method and apparatus for controlling power among modems in a multi-mode mobile communication device
US20110124303 *Jul 17, 2008May 26, 2011Alain HuotSystem for monitoring and controlling the power of a radio frequency (rf) signal in a short-range rf transmitter
US20160164495 *Dec 8, 2014Jun 9, 2016Honeywell International Inc.Auto-tunable antenna devices
CN100483931CSep 22, 2003Apr 29, 2009哈曼国际工业有限公司Heat protective system of amplifier output stage
CN100505521CDec 10, 2003Jun 24, 2009Nxp股份有限公司Amplifier circuit and method for preserving linearity of an power amplifier
DE10044452A1 *Sep 8, 2000Apr 4, 2002Infineon Technologies AgVerstärkerschaltung
EP0159670A2 *Apr 18, 1985Oct 30, 1985Nec CorporationRadio communication device and method of controlling transmitter output power
EP0159670A3 *Apr 18, 1985Oct 8, 1986Nec CorporationRadio communication device and method of controlling transmitter output power
EP0404294A1 *Mar 21, 1990Dec 27, 1990Nokia Mobile Phones (U.K.) LimitedLevelling control circuit
EP1337038A2 *Feb 14, 2003Aug 20, 2003Alcatel Alsthom Compagnie Generale D'electricitePower control loop, radiofrequency amplifying circuit, and transmitter for radiofrequency signals having such a circuit
EP1337038A3 *Feb 14, 2003Jan 4, 2006TCL & ALCATEL MOBILE PHONES LIMITEDPower control loop, radiofrequency amplifying circuit, and transmitter for radiofrequency signals having such a circuit
WO1998048520A1 *Apr 9, 1998Oct 29, 1998Ericsson Inc.Vswr control technique for terminal products with linear modulation
WO1999043096A1 *Dec 18, 1998Aug 26, 1999Motorola Inc.Data communications terminal and method of adjusting a power signal generated therefrom
WO2003055084A1 *Oct 16, 2002Jul 3, 2003Qualcomm, IncorporatedAdaptive transmitter power amplifier
WO2004054095A1 *Dec 10, 2003Jun 24, 2004Koninklijke Philips Electronics N.V.Preserving linearity of an isolator-free power amplifier by dynamically switching active devices
WO2004054096A1 *Dec 10, 2003Jun 24, 2004Koninklijke Philips Electronics N.V.Preserving linearity of an isolator-free power amplifier by dynamically adjusting bias and supply of active devices
WO2013000451A3 *Jun 18, 2012Feb 21, 2013Tesat-Spacecom Gmbh & Co. KgMethod and device for protecting a high-frequency power amplifier against a termination fault
Classifications
U.S. Classification330/298, 330/207.00P, 455/117
International ClassificationH03G3/30, H03F1/52
Cooperative ClassificationH03F2200/204, H03G3/3042, H03F1/52
European ClassificationH03F1/52, H03G3/30D2